Ultra wideband (UWB) systems have been developed to increase the bit-rate in wireless communication systems or, alternatively, to reduce the peak spectral power of the transmitted signals. In UWB systems the signal is transmitted as short pulses over a wide range of frequencies. As an example the pulse width should either be 15% of the carrier frequency, or, alternatively, have an absolute band width of 500 MHz. Typically the pulses should have a sufficiently low peak spectral power level that they do not interfere with other wireless techniques.
In such systems, some sort of oscillator is normally employed in order to generate the high frequency carrier signal. Depending on the specific application, different oscillator topologies, such as Dynatron, Hartley or Colpitt, Armstrong, Clapp, Wien bridge, relaxation or modifications thereof, may be used.
Irrespective of the topology used for the oscillator, the main operating principle is to keep a tank circuit, e.g. an LC circuit or any other combination of reactive elements, oscillating during a period of interest, which may be a longer continuous time period as in the case of radio broadcasts, or intermittent short time periods (wavelets) as in the case of e.g. impulse radio communications. As is well known, a tank circuit, or the oscillator as a whole, includes losses which are normally represented in the circuit in the form of one or more resistive elements. This implies that energy which is transferred back and forth between the reactive elements in the tank circuit will be dissipated in the resistive elements and the oscillation will eventually decay.
In order to sustain oscillation in the tank circuit, the losses need to be compensated for in some way. One way of providing sustained oscillation is to implement the oscillator as an amplifier which is provided with a positive feedback loop at the frequency of interest. Oscillation will in such a circuit be sustained as long as the so called Barkhausen criterion is fulfilled.
Another approach to provide a sustained oscillation in a tank circuit is to include a negative differential conductance (NDC) in the circuit in order to compensate for the inevitable losses present in the circuit. The NDC may be implemented in many different ways. For low frequency applications (<1 MHz) the NDC may e.g. be realized by means of an operational amplifier with a positive feedback loop. For high-frequency applications (>1 GHz) better performance is normally obtained by implementing the NDC in the form of a correctly biased resonant tunnel diode (RTD).
To increase the understanding of the latter approach, FIG. 1 illustrates an example of a characteristic curve of an RTD compared to that of a standard PN junction diode.
In the standard PN junction diode conduction occurs only if the voltage applied to its terminals is large enough to overcome the potential barrier of the PN junction. Thus, the current-voltage characteristics of the standard PN junction diode exhibits a positive resistance/conductance irrespective of the biasing voltage applied as indicated by the dashed curve in FIG. 1.
On the other hand, an RTD exhibits an unusual current-voltage characteristics as compared with that of an ordinary PN junction diode. The characteristic curve for an RTD is indicated by the solid line in FIG. 1. The three most important aspects of this curve are i) that the forward current initially increases 150 to a local maximum (peak) 110 as the forward bias voltage increases from zero volts, ii) that the forward current, after passing the local maximum 110, decreases 120 to a local minimum (valley) 130 as the forward bias voltage increases, and finally iii) that the forward current once again, after passing the local minimum 130, increases 140 as the forward bias voltage increases. Thus, as can be inferred from FIG. 1, the RTD exhibit a negative differential conductance at the portion 120 of the characteristic curve when the current decreases with an increasing forward bias voltage. Similarly, the RTD exhibit a positive differential resistance at the portions 140, 150 of the characteristic curve when the current increases with an increasing forward bias voltage.
As mentioned above, in UWB systems the signal is transmitted as short pulses (wavelets). To this end it is desirable to control the magnitude of the NDC thereby enabling an oscillation signal to be provided to a radiating element during a desired time period (i.e. for the duration of the short pulses).
Prior art solutions for providing short high-frequency pulses have been provided in which a gated tunnel diode is formed by the embedding of a metal gate in the direct vicinity of a double barrier resonant tunnel diode. The NDC of the gated tunnel diode is in accordance with the disclosure above used to compensate for the losses in the tank circuit (or in the oscillator circuit as a whole) and hence provide a sustained oscillation in the tank circuit. The bias applied to the gate controls the magnitude of the NDC in the circuit and may be used to turn the oscillator on and off.
Even though the gated tunnel diode may be used for providing wavelets, the internal gating required in the gated tunnel diode induces constraints with respect to the flexibility in wavelet generation. Additionally, the diode current through a gated tunnel diode cannot be turned off completely even when the gate is set to a very negative value.
Thus, a more efficient and flexible solution for providing wavelets is desired.